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common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts.

Properties and Classification

Properties of the clays include plasticity, shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, and capacity of the surface to take decoration. On the basis of such qualities clays are variously divided into classes or groups; products are generally made from mixtures of clays and other substances. The purest clays are the china clayschina clay,
one of the purest of the clays, composed chiefly of the mineral kaolinite usually formed when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin
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 and kaolins. "Ball clay" is a name for a group of plastic, refractory (high-temperature) clays used with other clays to improve their plasticity and to increase their strength. Bentonites are clays composed of very fine particles derived usually from volcanic ash. They are composed chiefly of the hydrous magnesium-calcium-aluminum silicate called montmorillonite. See also fuller's earthfuller's earth,
mineral substance characterized by the property of absorbing basic colors and removing them from oils. It is composed mainly of alumina, silica, iron oxides, lime, magnesia, and water, in extremely variable proportions, and is generally classified as a
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Individual clay particles are always smaller than 0.004 mm. Clays often form colloidal suspensions when immersed in water, but the clay particles flocculate (clump) and settle quickly in saline water. Clays are easily molded into a form that they retain when dry, and they become hard and lose their plasticity when subjected to heat.


Clays are divided into two classes: residual clay, found in the place of origin, and transported clay, also known as sedimentary clay, removed from the place of origin by an agent of erosion and deposited in a new and possibly distant position. Residual clays are most commonly formed by surface weathering, which gives rise to clay in three ways—by the chemical decomposition of rocks, such as granite, containing silica and alumina; by the solution of rocks, such as limestone, containing clayey impurities, which, being insoluble, are deposited as clay; and by the disintegration and solution of shale. One of the commonest processes of clay formation is the chemical decomposition of feldsparfeldspar
or felspar
, an abundant group of rock-forming minerals which constitute 60% of the earth's crust. Chemically the feldspars are silicates of aluminum, containing sodium, potassium, iron, calcium, or barium or combinations of these elements.
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Clay consists of a sheet of interconnected silicates combined with a second sheetlike grouping of metallic atoms, oxygen, and hydroxyl, forming a two-layer mineral such as kaolinitekaolinite
, clay mineral crystallizing in the monoclinic system and forming the chief constituent of china clay and kaolin. It is a hydrous aluminum silicate commonly formed by the weathering and decomposition of rocks containing aluminum silicate compounds; feldspar is a chief
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. Sometimes the latter sheetlike structure is found sandwiched between two silica sheets, forming a three-layer mineral such as vermiculite. In the lithification process, compacted clay layers can be transformed into shale. Under the intense heat and pressure that may develop in the layers, the shale can be metamorphosed into slate.


From prehistoric times, clay has been indispensable in architecture, in industry, and in agriculture. As a building material, it is used in the form of brickbrick,
ceramic structural material that, in modern times, is made by pressing clay into blocks and firing them to the requisite hardness in a kiln. Bricks in their most primitive form were not fired but were hardened by being dried in the sun.
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, either sun-dried (adobe) or fired. Clays are also of great industrial importance, e.g., in the manufacture of tiletile,
one of the ceramic products used in building, to which group brick and terra-cotta also belong. The term designates the finished baked clay—the material of a wide variety of units used in architecture and engineering, such as wall slabs or blocks, floor pavings,
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 for wall and floor coverings, of porcelain, china, and earthenware, and of pipe for drainage and sewage. Highly absorbent, bentonite is much used in foundry work for facing the molds and preparing the molding sands for casting metals. The less absorbent bentonites are used chiefly in the oil industry, e.g., as filtering and deodorizing agents in the refining of petroleum and, mixed with other materials, as drilling muds to protect the cutting bit while drilling. Other uses are in the making of fillers, sizings, and dressings in construction, in clarifying water and wine, in purifying sewage, and in the paper, ceramics, plastics, and rubber industries.

Clay as a Soil

Clay is one of the three principal types of soil, the other two being sand and loam. A certain amount of clay is a desirable constituent of soil, since it binds other kinds of particles together and makes the whole retentive of water. Excessively clayey soils, however, are exceedingly difficult to cultivate. Their stiffness presents resistance to implements, impedes the growth of the plants, and prevents free circulation of air around the roots. They are cold and sticky in wet weather, while in dry weather they bake hard and crack. Clods form very often in clayey soils. Clays can be improved by the addition of lime, chalk, or organic matter; sodium nitrate, however, intensifies the injurious effects. In spite of their disadvantages, the richness of clay soils makes them favorable to the growth of crops that have been started in other soil.


See R. E. Grim, Clay Mineralogy (2d ed. 1968); R. W. Grimshaw, The Chemistry and Physics of Clays and Allied Ceramic Materials (4th ed. 1971).


A natural, earthy, fine-grained material which develops plasticity when mixed with a limited amount of water; composed primarily of silica, alumina, and water, often with iron, alkalies, and alkaline earths.
The fraction of an earthy material containing the smallest particles, that is, finer than 3 micrometers.
A special grade of absorbent clay used as a filtering medium in refineries for removing solids or colorizing matter from lubricating oils.


A fine-grained, cohesive, natural earthy material; plastic when sufficiently wet; rigid when dried; vitrified when heated in a kiln to a sufficiently high temperature; used in making brick, as wall infilling, and as daub in wattle-and-daub.


1. a very fine-grained material that consists of hydrated aluminium silicate, quartz, and organic fragments and occurs as sedimentary rocks, soils, and other deposits. It becomes plastic when moist but hardens on heating and is used in the manufacture of bricks, cement, ceramics, etc.
2. Poetic the material of the human body


Henry. 1777--1852, US statesman and orator; secretary of state (1825--29)
References in periodicals archive ?
FOTHERINGHAM: In the southwest corner of this county, there is a deposit of calcium carbonate in a clayish form that has been accepted as a solution to soak up the oil in the gulf.
Intermountain valleys and low plains were composed mainly by clayish dark black or dark gray, pelic vertisols, and reddish and clayish cromic vertisols, whereas calcic xerosols prevail in the high plains.
We are just a few hours away from Huehuetenango and the narrow valleys have given way to wide open spaces with clayish soils in a palette of colors varying from orange to lavender.
The underlying 91 m thick unit is mainly composed by clay and clayish sand.
2) is: (i) a resistive layer (500-1700 ohm-m) with variable thickness and moisture content at the surface, which consists of a mixture of gravel, silt, and sand; (ii) a thick clay and clayish sand layer having resistivity values of 30 to 40 ohm-m; and (iii) a saturated sandy layer with saline water having resistivity values of 1 to 10 ohm-m.
3b) is composed by (1) a thin layer of medium to coarse grained sand and gravel (surface layer) with an apparent resistivity of about 250 to 1300 ohm-m, (2) a second fine to medium grain size sand and gravel layer (100-500 ohm-m), and (3) a relatively low-resistivity third layer (20-100 ohm-m) indicating a clay and clayish sand formation.
From the top to the bottom, these are: (1) a sandy, clayish sand, and gravel of fine grained size having apparent resistivity of 60 to 250 ohm-m, followed by (2) the presence of clay and clayish sand formation (40-80 ohm-m), a (3) saturated sand with good groundwater quality (100 to 200 ohm-m), (4) a clay and clayish sand formation (40-80 ohm-m), and (5) a sandy formation saturated with fresh groundwater (80-200 ohm-m).
3d) describe qualitatively a model composed by (1) a relatively thin surface layer with an apparent resistivity of 200 to 1300 ohm-m of medium to coarse grained material with a variable moisture content, followed by (2) the presence of a relatively low-resistivity second layer (40-80 ohm-m), indicating a clay and clayish sand formation, and (3) a sandy formation saturated with fresh groundwater (100-200 ohm-m).
In general, the layer thickness increases with increasing distance from the coast, followed by the presence of a relatively conductive second clay and clayish sand layer, ranging from 20 to 100 ohm-m, at shallower depths and thickening landwards.
The first zone shows resistivity values in the range 500-600 ohm-m at the surface, considered to have been caused by coarse grained sand, gravel and sand dune at 10-20 m depth, followed by the presence of a relatively conductive second clay and clayish sand layer, ranging from 30 to 90 ohm-m, at shallower depths and thickening landwards, followed by conductive layer with resistivities of 2 to 3 ohm-m corresponding to saline water.